Hearing aids generally use conventional zinc-air button batteries because of their inherent high-energy storage and convenience. The zinc-air battery is commercially available in several different sizes. All are pancake looking with the diameter several times the height. The smallest button battery is for the in-the-canal hearing aids. This form factor is not always suitable for the orientation needed in the ear so that this becomes increasingly important as hearing aids become smaller.
A zinc-air battery system consists of four basic elements, i.e., the anode, the cathode, the air cathode and the electrolyte. In a zinc-air system, the cathode is the oxygen in the air and the air cathode promotes the reaction of oxygen with the electrolyte. The electrolyte is a liquid, generally potassium hydroxide. The anode is generally a metal, such as zinc. Since zinc-air batteries are exposed to air for activation, the system's electrolyte is depleted by evaporation. While evaporation can be controlled to provide the limited life required, the battery must be sealed during storage. With traditional hearing aid batteries, a metal foil with pressure sensitive adhesive is applied over small holes in the battery so as to prevent the entry of air. When, the battery is to be used, the foil is removed allowing air to enter the battery and the reaction begins.
Hearing aids have been made smaller to provide for in-the-ear and completely in the canal hearing aids. The smaller the hearing aid the smaller the battery must be. Because of the small sizes of batteries being used in the hearing aids, it is difficult and frustrating to replace the batteries when the batteries run out. The small batteries are difficult to handle, especially for the elderly, who are the majority of the users of hearing aids.
To overcome the problem of the necessity of replacing batteries in a hearing aid, a disposable hearing aid has been developed. The disposable hearing aid is of a structure that is so inexpensive to manufacture that it is possible to merely replace the whole hearing aid, rather than just the battery, when the battery runs out. Thus, the life of a disposable hearing aid is dependent on the life of the battery. The longer the life of the battery, the less often the hearing aid has to be replaced and the less expensive it is to use the disposable hearing aid. Therefore, it would be desirable to have a battery for a disposable hearing aid that has a longer life.